Many variables determine your refund amount. I would guess from the situation you describe you had different amounts withheld from taxes throughout the year from your payroll.
Without seeing both tax returns, we could only speculate.
As Catina mentioned, there are many variables that play a factor in the refund amount. One of the biggest (as mentioned by Catina) are the withholding amounts. If your co-worker had more taxes taken out throughout the year on their checks, then as it gets reported to the irs during tax time, your coworker would get more back due to an over payment of taxes. Also keep in mind that other factors could play into it such as other deductions your coworker might be able to utilize or charitable contributions, ect.
The only way to tell exactly why would be to put your tax returns next to each other and compare them line by line. Or----ask him what was in box 2 of his W-2 and compare it to what you had in your box 2. Then compare what each of you had in box 17 of each of your W-2's.